Secretary of State John Kerry told his Russian counterpart on Saturday the United States was deeply concerned about reports that Moscow was moving toward a major military build-up in Syria widely seen as aimed at bolstering President Bashar al-Assad.

U.S. authorities have detected "worrisome preparatory steps," including transport of prefabricated housing units for hundreds of people to a Syrian airfield, that could signal that Russia is readying deployment of heavy military assets there, a senior U.S. official told Reuters.

The official, speaking on condition of anonymity, said Moscow's exact intentions remained unclear but that Kerry called Russian Foreign Minister Sergei Lavrov to leave no doubt of the U.S. position.

The State Department pointed to media accounts suggesting an "imminent enhanced Russian military build-up" in Syria.

"The secretary made clear that if such reports were accurate, these actions could further escalate the conflict, lead to greater loss of innocent life, increase refugee flows and risk confrontation with the anti-ISIL coalition operating in Syria," the State Department said, using an acronym for Islamic State.

Russia's actions could signal an intensified effort to support Assad, a longtime Russian ally who has seen the area he controls whittled down to a fifth or less of Syria's territory after more than four years of grinding civil war.

Among the latest steps by Russia is the delivery of the temporary housing units and a portable air traffic control station to an airfield near the port city of Latakia, an Assad stronghold, the U.S. official said.

The Russians have also filed military overflight requests with neighboring countries, the U.S. official said.

In addition, Russia has dispatched a military advance team to Syria, the New York Times reported.

It cited unnamed U.S. officials saying that while there was no indication Russia intended to send in significant ground forces, the housing could accommodate as many as 1,000 military advisers and other personnel and enable the airfield to be a supply hub or a launching pad for Russian airstrikes.

But the official told Reuters: "It is inconclusive exactly what the Russians' intent is. We have not seen the actual deployment of military assets or aircraft or forces."

The official said the conclusions were drawn from a "variety of sources." The Los Angeles Times reported U.S. intelligence gathered the evidence from satellite reconnaissance photos.

A U.S. security source also told Reuters there were signs of a Russian move to intervene in Syria beyond its already robust military support role, which have included weapons and training.

The source said the United States will be watching to see whether any increased Russian military role will be used strictly to help Assad or to push back Islamic State, which has seized swathes of Syria and neighboring Iraq and is the target of a U.S.-led coalition bombing campaign.

The U.S. official declined to say how Lavrov had responded to Kerry's concerns. The State Department said the two agreed that discussions on the Syrian conflict would continue this month in New York, where the U.N. General Assembly meets.

Lavrov said last month the United States should cooperate with Assad to fight Islamic State militants.

The United States and Russia have long been at loggerheads over Syria. Russia has backed Assad. The United States advocates a political transition to end his rule.